Flow control of exhaust gas through an internal combustion engine has been used in order to provide vehicle engine braking. Engine braking may include exhaust brakes, compression release type engine brakes, bleeder type engine brakes, and/or any combination thereof. The general principle underlying such brakes is the utilization of gas compression generated by the reciprocating pistons of an engine to retard the motion of the pistons and thereby help to brake the vehicle to which the engine is connected.
Exhaust brakes are known to be useful to help brake a vehicle. Exhaust brakes may generate increased exhaust gas back pressure in an exhaust system, including an exhaust manifold, by placing a restriction in the exhaust system downstream of the exhaust manifold. Such restriction may take the form of a turbocharger, an open and closeable butterfly valve, or any other means of partially or fully blocking the exhaust system.
By increasing the pressure in the exhaust manifold, an exhaust brake also increases the residual cylinder pressure in the engine cylinders at the end of the exhaust stroke. Increased pressure in the cylinders, in turn, increases the resistance encountered by the pistons on their subsequent up-strokes. Increased resistance for the pistons results in braking the vehicle drive train which may be connected to the pistons through a crank shaft.
In some known vehicle braking systems, exhaust brakes have been provided such that the restriction in the exhaust system is either fully in place or fully out of place. These exhaust brakes may produce levels of braking which are proportional to the speed of the engine (RPM) at the time of exhaust braking. The faster the engine speed, the greater the pressure of the gas in the exhaust manifold and cylinders. The higher pressure results in increased resistance to the up-stroke of the piston in the cylinder and therefore, increased braking.
Because the exhaust system and engine cannot withstand unlimited pressure levels, many systems include exhaust brake restrictions that are designed such that their operation at a rated maximum engine speed will not produce unacceptably high pressures in the exhaust system and/or engine that exceed a pressure limit. At engine speeds below the rated maximum engine speed, however, these exhaust brake restrictions may produce pressures that are lower than necessary. As a result, less than optimum braking may occur below the rated maximum engine speed.
In some known vehicle braking systems, exhaust brakes have been provided with a butterfly valve having a fixed-sized opening, or orifice, formed in the valve. When the valve is closed, the orifice provides an exhaust gas flowpath through the valve. The orifice may be sized such that at the rated maximum engine speed, the orifice permits a sufficient release of pressure from the upstream side of the valve that the exhaust pressure does not exceed the pressure limit for the engine. FIG. 1 is a graph illustrating retarding power and back pressure versus engine speed (RPM) for an exhaust brake system having a valve and an orifice. The graph also illustrates an exhaust pressure limit and a targeted retarding power for a particular engine over a range of engine speeds. It is to be understood that FIG. 1 is for exemplary purposes only, and the relative values for retarding power and exhaust back pressure may vary depending on a variety of factors, such as, for example, the specifications of the vehicle engine.
With reference to FIG. 1, when the exhaust brake is activated the butterfly valve closes and exhaust pressure is generated upstream of the valve. If the exhaust brake is operated without the orifice, or with the orifice in a fully closed position (closed orifice), increased exhaust pressure, and, correspondingly, increased retarding power may result. At low to mid-range engine speeds (shown generally to the left of the heavy vertical line in FIG. 1), the exhaust brake with closed orifice generates exhaust back pressure that is below the engine pressure limit. At higher engine speeds (shown generally to the right of the heavy vertical line in FIG. 1), however, the exhaust brake in the fully closed orifice position may produce unacceptably high exhaust pressures. When the exhaust brake is operated with the orifice in an open position (fixed orifice), the generated exhaust back pressure remains below the pressure limit, even at higher engine speeds. However, because lower exhaust pressures are generated, less than optimal retarding powers may be achieved. Thus, what is needed is an exhaust brake system and method adapted to optimize engine retarding power by maintaining exhaust pressure at higher engine speeds substantially near the exhaust pressure limit, without exceeding that limit.
In some known vehicle braking systems, exhaust brakes have been provided with variable restriction. These variable restrictions may be designed such that their operation is dependant on a predetermined back pressure level, not the rated maximum speed. Because the restriction is not dependent on the rated maximum speed, improved braking may occur below this speed.
Some variable restriction exhaust brake systems may include a spring loaded pressure-relief valve operable to admit flow of exhaust gases along a bypass flowpath only when a prescribed back pressure is reached. When the prescribed back pressure is reached, the pressure overcomes the force of the valve spring and opens the valve to relieve the pressure. When the valve opens, however, the flow of the gas through the valve may create a localized dynamic pressure drop near the valve. This pressure drop may cause the valve to close prematurely, or to rapidly close and then reopen. As a result the desired level of exhaust back pressure may not be easily maintained, and the desired level of braking may not be achieved.
Embodiments of the present invention may provide apparatus and methods for controlling exhaust pressure in an internal combustion engine. Some embodiments of the present invention may provide controlled exhaust gas back pressure to optimize one or more engine valve events, such as, for example, engine braking. Some embodiments of the present invention may control exhaust gas back pressure independent of the effect of dynamic pressure on means for controlling the exhaust pressure. Advantages of embodiments of the invention are set forth, in part, in the description which follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.